Warn against using `-f mp4` (#13915)

Authored by: seproDev

yt_dlp/__init__.py

@@ -500,6 +500,14 @@ def validate_options(opts):
             'To let yt-dlp download and merge the best available formats, simply do not pass any format selection',
             'If you know what you are doing and want only the best pre-merged format, use "-f b" instead to suppress this warning')))
 
+    # Common mistake: -f mp4
+    if opts.format == 'mp4':
+        warnings.append('.\n         '.join((
+            '"-f mp4" selects the best pre-merged mp4 format which is often not what\'s intended',
+            'Pre-merged mp4 formats are not available from all sites, or may only be available in lower quality',
+            'To prioritize the best h264 video and aac audio in an mp4 container, use "-t mp4" instead',
+            'If you know what you are doing and want a pre-merged mp4 format, use "-f b[ext=mp4]" instead to suppress this warning')))
+
     # --(postprocessor/downloader)-args without name
     def report_args_compat(name, value, key1, key2=None, where=None):
         if key1 in value and key2 not in value: